The invention relates to a bipolar electrode for medical applications, particularly an implantable heart pacemaker electrode, comprising an insulated conductor system, at least one active electrode and a passive electrode disposed along the conductor system at an interval from said active electrode.
In the case of unipolar heart pacemaker treatment, problems often occur with muscle stimulations and/or muscle inhibitions at the housing of the heart pacemaker, said housing usually representing the passive electrode in the electrode system. A possible solution of this problem, as known, resides in utilizing a bipolar electrode system, i.e. of disposing the passive electrode in the proximity of the active electrode(s) inside of the heart. There is an effort to reduce the impedance of the electrode system due to the small distance between the active and passive electrodes. Known passive electrodes consist, for example, of a cylindrical body of a platinum/iridium alloy having a surface area of approximately 50 mm.sup.2. Due to the relatively low double layer capacitance of platinum/iridium (10 .mu.F/cm.sup.2, 1 kHz), this electrode must have such a large surface area in order to keep the polarization losses within justifiable limits.
Considerable mechanical problems arise, however, due to the large dimensions of the passive electrode. Imagining, for example, an insulated electrical conductor 3 mm in diameter, then the cylindrical body of the passive electrode must be about 5 mm long in order to have the required surface area. That produces a considerable stiffening of the otherwise extremely flexible electrical conductor in the proximity of the active electrode. When, for example, it is a matter of a heart ventricle electrode which is to be applied in the tip of the left heart ventricle, then the passive electrode likewise lies in this heart ventricle. Given the large number of bends that this electrical conductor is exposed to, such a pronounced stiffening represents a geat burden that increases the risk that damage to the insulation or a break of the conductor will occur in the proximity of the stiffening.